The invention relates to a level-measuring device for determining the level of a medium in a container.
Known level-measuring devices use for this purpose a transit time method, for example a TDR method with guided pulses, or another pulse echo method by means of radiation in the direction of the medium. In this method, a pulse-shaped electromagnetic signal is used which propagates at the velocity of light. The medium surface represents an irregularity for the propagation of this signal and the signal is thus reflected to the transmitter. To be able to determine the transit time of the reflected transmit signal, a time measurement must be performed in the nanosecond range. Since, as a rule, electronic circuits operate too slowly for a direct transit time measurement in this case, the echo signal variation must be transformed into an extended time scale in a suitable manner. For this purpose, a pulse is periodically sent out from which a signal sample is taken by a sampling circuit at a position which progresses from pulse to pulse.
After a measuring cycle, which can consist of several thousand individual periods, has elapsed, a time-transformed signal variation is obtained which represents the total transit path of the signal. From this, the distance between medium surface and level-measuring device can be determined, which is a measure of the level. Industrially used measuring ranges extend from a few 10 cm up into ranges of over 50 m.
Known level-measuring devices of this type comprise a signal generating device for generating both a radio-frequency transmit signal and a sampling control signal for sampling the radio-frequency signal, a transmitting and/or receiving device and a sampling device, controlled by the sampling control signal, for sampling the radio-frequency signal, the signal generating device, the transmitting and/or receiving device and the sampling device being connected by a coupling device. To be able to meet the requirements of the various measuring ranges, signal generator devices and sampling circuits have been varied in previous measuring devices. However, this results in unwanted losses in the connection between signal generating device and transmitting and/or receiving device and between transmitting and/or receiving device and sampling device.
It is, therefore, an object of the present invention to avoid the above-mentioned disadvantages and, in particular, to create level-measuring devices in which signal generator devices and sampling devices and transmitting and/or receiving devices are optimally connected to one another.
This object is achieved by a first variant of the invention in which the level-measuring device exhibits a signal generating device for generating both a radio-frequency transmit signal and a sampling control signal for sampling the radio-frequency signal, a transmitting and/or receiving device and a sampling device, controlled by the sampling control signal, for sampling the radio-frequency signal, the signal generating device, the transmitting and/or receiving device and the sampling device being connected by a coupling device which comprises a transformer.
This object is also achieved by a second variant of the invention in which the level-measuring device exhibits a signal generating device for generating both a radio-frequency transmit signal and a sampling control signal for sampling the radio-frequency signal, a transmitting and/or receiving device and a sampling device, controlled by the sampling control signal, for sampling the radio-frequency signal, the signal generating device, the transmitting and/or receiving device and the sampling device being connected by a coupling device which comprises at least one line node.
This object is also achieved by a third variant of the invention in which the level-measuring device exhibits a signal generating device for generating both a radio-frequency transmit signal and a sampling control signal for sampling the radio-frequency signal, a transmitting and/or receiving device and a sampling device, controlled by the sampling control signal, for sampling the radio-frequency signal, the signal generating device, the transmitting and/or receiving device and the sampling device being connected by a coupling device which comprises a xcex/4 coupler.
In addition, this object is achieved by a fourth variant of the invention in which the level-measuring device exhibits a signal generating device for generating both a radio-frequency transmit signal and a sampling control signal for sampling the radio-frequency signal, a transmitting and/or receiving device and a sampling device, controlled by the sampling control signal, for sampling the radio-frequency signal, the signal generating device, the transmitting and/or receiving device and the sampling device being connected by a coupling device which comprises a circulator.
Preferred embodiments of the third variant of the level-measuring device according to the invention relate to those with xcex/4 couplers which are Wilkinson couplers, Lange couplers, directional couplers, line couplers or, for example, coaxial couplers.
In other preferred embodiments of the invention, the signal generating device comprises a comparator circuit, a beat oscillator circuit or a regulated delay circuit which are used for delaying the sampling signal with respect to the radio-frequency transmit signal.
Other preferred embodiments of the invention relate to level-measuring devices with sampling devices which comprise a diode ring circuit or at least one diode and one holding capacitor or a mixer circuit.
In yet other preferred embodiments of the invention, the radio-frequency transmit signal is a burst signal.
The invention enables transmitting and receiving characteristics of the level-measuring device to be matched to one another in a simple manner for different signals and/or measuring ranges by optimizing one of its components.
In addition, the invention uses a simple and rugged DC decoupling of the transmitting and/or receiving device from the rest of the measuring device so that the level-measuring device according to the invention is particularly suitable for uses in hazardous areas or applications.
In the text which follows, the invention will be explained and described in greater detail using the example of various preferred embodiments, using and referring to the attached drawings. Components or assemblies of identical construction or identical in their operation are provided with the same reference symbols for the sake of simplicity.